Terminal or user equipment in a communications system may perform measurements on frequencies different from its actual sending/receiving frequency in order to observe measurements from devices such as base stations or to perform OTD (Observed Time Difference) measurements. During this time (transfer gap) no data transfer takes place. To maintain an average data transfer rate, the data rate outside of the transfer gap is increased in certain time frames. The operational mode in these time frames is referred to as compressed mode. This compressed mode influences the data transfer, and creates interdependencies among various data.
A base station is typically defined as a central unit in a cellular communications network, that serves terminals or user equipments within a cell of the communications network. Typically, it comprises at least a sending/receiving unit. In UMTS it is often referred to as node B.
This can be described by using an example regarding a UMTS System:
During a connection that is established between a communication device or user equipment (UE) and a base station (BS or Node B), the user equipment may also observe other base stations in order to find out the base station the optimum connection can be installed to.
For observing another base station, the user equipment has to tune in on frequencies distinct from its actual sending/receiving frequencies. Thus during the time the user equipment is observing other frequencies, no data is being transmitted or received, at least if the user equipment has only one synthesizer and/or only one RF-part (RF: Radio Frequency).
However, the user of the user equipment should not notice under normal working operation, that the data transfer has been disrupted in order to create transfer gaps for the so called “inter-frequency measurements” by which frequencies distinct from the actual sending/receiving frequency or frequencies are observed. In the framework of the UMTS standardization, this item is dealt with in [1].
To maintain a constant average data rate in the presence of transfer gaps, the net data transfer rate is increased before and after the transfer gaps. “Net data” in this context refers to data actually carrying information. A predetermined overhead is added to the net data to ensure that the data can be decoded correctly at the receiver, even if the transmission has not been ideal, i.e. experiences some degradation. The overall data is referred to as gross data, wherein the overhead of data may consist of parity bits originating from channel coding. Data transfer may be either the transmission or the reception of data or both.
An illustration of a transfer disrupted by a transfer gap TG, e.g. a transmission gap, is shown in FIG. 1, which is taken from [1]: The transmit power is depicted versus time, wherein the latter is segmented in frames F as time intervals, each frame itself contains several time slots. The frames during which the user equipment listens to another base station and thus cannot be transferring data continuously are referred to as compressed frames, as the transfer rate has to be increased in some timeslots in this frame to achieve an average rate similar to normal frames, that is when the compressed mode is off.
The frames, in which the data are transferred compressed, are referred to as compressed frames, and the respective operating mode as compressed mode.
In compressed frames, TGL (transmission gap length) slots from a first slot Nfirst to a last slot Nlast are not used for transmission of data. As illustrated in FIG. 1, the instantaneous transmit power P, which is depicted versus time t, is increased in the compressed frame Fc, before and after the transmission gap TG with the length TGL in order to keep the quality (the BER (Bit Error Rate) or the FER (Frame Error Rate)) unaffected by the reduced processing gain. F denotes the length of a normal frame. By “reduced processing gain” it is meant that the data is encoded less safe than during “normal transmission”. The amount of power increase depends on the actually used transmission time reduction method (see [1], subclause 4.4.3).
In FIG. 2 an ordinary transmission sequence can be seen, which is used to explain the terms demodulation, coding etc.
The signal may be generated at the source or transmitter TX. In a subsequent analog to digital converter A/D the signal is digitized, thus the smallest information carrying unit is one bit. Digitizing includes the steps of sampling and quantizing the signal. Then various coding steps in the encoder C are performed: source coding is performed to get rid of redundancies in the signal or digitized data are used directly (which means no A/D converting or source coding etc needs to be done); channel coding is applied to protect the bits. After coding the signal is spread. At this point the smallest information carrying unit is a chip. Due to spreading the chip rate for a transmission is typically considerably higher than the bit rate.
At the digital modulator DM the data is ‘translated’ into symbols that differ for the various modulation and coding schemes. The higher a modulation the higher the number of bits that are translated into a symbol.
Next the data being transferred may be subject to influences from noise and interference that can have an impact on the data. For example a previous symbol (1,1) at the digital demodulator might be changed to (0.7,0.9). Hence, the transfer is referred to via an analogous channel AC. At the receiving side the corresponding processes of demodulation at the digital demodulator DD and the decoding at the Decoder D and the digital to analog conversion at the D/A converter are taking place.
Generally speaking, in compressed mode the transmit power is increased to ensure a safe transmission of the less safe encoded data: By coding the data less, with the same gross data transfer rate a higher net data transfer rate can be achieved. The data bits are preferably punctuated more than in the frames before or the coding of the data bits has been performed with a lower spreading factor. The compressed mode therefore entails rather complex calculations how the gross data are modified                depending on the gap length        and on the current data transfer rate        and on the duration of the compressed mode (cf. FIG. 1, the time required for the time slots with the higher transmitting power)        and in how this modification is realized, e.g. by using a different modulation scheme        a different spreading factor puncturing of data, i.e. cutting out individual or group of bits.        
It is decided by the network which frames are compressed. When in compressed mode, compressed frames can occur periodically or requested on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements. In OSI-layers above the physical layer the knowledge of the scheduling of the compressed frames is existent, thus the above mentioned calculations for the compressed mode can be done. As a further variant for the realization of compressed frames, it is known that higher layers can also restrict the data rate during frames which will undergo compression on the physical layer, thus making the operation in compressed mode more reliable because less excessive rate matching will be necessary for the compressed frames due to the lower data-rate.
Furthermore, transmission gaps can be placed at different positions, depending on factors such as interfrequency power measurement, acquisition of control channel of another system or carrier and handover operation, cf. [1], section 4.4.4. For the so called single frame method, the transmission gap is located within the compressed frame. The exact position depends on the length of the transmission gap TGL (transmission gap length). For the double frame method the transmission gap is overlaps two neighbored frames. In the example of FIG. 3a (discussed in greater detail below), the single frame method is shown, in FIG. 3b an example for the double frame method.
For example, this type of compressed mode is applied in UMTS (Universal Mobile Telecommunications System) to the DPDCH (Dedicated Physical Data Channel), across which data are transferred by circuit switching. As explained above, the methods for compressing the frames require rather complex calculations.
Furthermore, packet switched transfer modes, which may be operated in parallel to the circuit switched modes or continuous channels via which data, such as speech can be transferred, are affected by the transmission gaps TG. This will be detailed below: In a packet switched transfer mode, the data is segmented into packets. Each packet is transferred individually. The quality of the reception is decided on basis of various data operations such as demodulation or decoding (cf. FIG. 2). The receiver sends back a receipt of the reception, e.g. an ‘ACK’ (Acknowledge) or a ‘NACK’ (Not acknowledge) depending whether it has recognized a packet as been received correctly or not. A channel with packet switched transfer mode such as in UTMS, the HS-DSCH (High Speed Downlink Shared Channel) is mapped to the physical channel HS-PDSCH. An overview of this technique is provided in [3].
The HSDPA data channel may be viewed as an enhancement of the existing UMTS.downlink shared-channel (DSCH). HSDPA allows to code multiplex different users or mobile stations with spreading factor of up to 16 codes. The primary multiple access, however, is in the time domain, where different users can be scheduled every transmission time interval (TTI), which corresponds to 3 UMTS slots, i.e., 2 ms. Also the number of codes allocated to one user can change from TTI to TTI. Depending on the system load and channel conditions, the base station or Node B adapts modulation and code rate for each user A predetermined combination of code rate and modulation is referred to as MCS (Modulation and Coding Scheme) level. The MCS level may change every TTI. It is determined by the base station based on feedback information or channel quality information (CQI) from the user terminal or mobile station, which stems from channel condition measurements. The channel quality information is sent with a periodicity ranging from one to 80 TTIs.
To achieve the high data rates, modulation and coding schemes are used which allow a high information bit rate per code. Therefore so called “higher modulation” techniques are used by which a symbol contains more than 2 bits. One example is 16-CAM (Quadrature Amplitude Modulation). For these modulation techniques, the individual positions for a bit within a symbol are not equally protected. Therefore, there is the effect of mapping important bits to well protected positions and less important bits to less protected positions. This is referred to as bit priority mapping and will be detailed below using an example from HSDPA. Furthermore, for channel coding so called “turbo codes” with rate R=⅓ are used. The rate indicates the ratio of the total number of bits to the number of load or systematic bits.
The HS-DSCH is shared among several users. The respective transfer rate for each of the users is decided on basis of the individual channel quality. One of the multiple access possibilities is in the time domain, where different users can be scheduled every transmission time interval (TTI), which corresponds to three UMTS slots (UMTS: Universal Mobile Telecommunication System), or 2 ms.
The transport channel HS-DSCH is mapped—as mentioned above—to the physical channel HS-PDSCH (High Speed Physical Downlink Shared Channel), to which a compressed mode can be applied. For higher data rates, where a single HS-PDSCH cannot carry the entire data rate, a set of HS-PDSCH channels can be used, in this case all the HS-PDSCHs of the set are transmitted simultaneously and they can be distinguished because they use different spreading codes. However, the invention as described herein should not be affected whether one HS-PDSCH or a set is used.
In principle the above described compressed mode can be applied to packet switched data, too. Therefore calculations need to be done as described in the references below (cf. [2]). However, a simpler process is desirable to make the calculations less complex.